Human fibroblast interferon will be purified on concanavalin A-agarose and by hydrophobic chromatography. The purity of the preparation will be evaluated by SDS-PAGE. Preparative isoelectrofocusing will be added, if necessary, for a further purification. The nature of the heterogeneity of human leukocyte interferon preparations will be probed by molecular sieving, SDS-PAGE and biological characterization. BIBLIOGRAPHIC REFERENCES: Binding of human interferons to immobilized Cibacron Blue F3GA: the nature of molecular interaction. W. J. Jankowski, W. von Muenchhausen, E. Sulkowski and W. A. Carter. Biochemistry 15, 5182-5187, 1976. Hydrophobic interaction of human, mouse and rabbit interferons with immobilized hydrocarbons. M. W. Davey, E. Sulkowski and W. A. Carter. J. Biol. Chem. 251, 7620-7625, 1976.